use std::collections::VecDeque;
use std::time::Duration;

use serde::Deserialize;

use crate::error::ApiError;
use crate::sse::SseParser;
use crate::types::{MessageRequest, MessageResponse, StreamEvent};

const DEFAULT_BASE_URL: &str = "https://api.anthropic.com";
const ANTHROPIC_VERSION: &str = "2023-06-01";
const REQUEST_ID_HEADER: &str = "request-id";
const ALT_REQUEST_ID_HEADER: &str = "x-request-id";
const DEFAULT_INITIAL_BACKOFF: Duration = Duration::from_millis(200);
const DEFAULT_MAX_BACKOFF: Duration = Duration::from_secs(2);
const DEFAULT_MAX_RETRIES: u32 = 2;

#[derive(Debug, Clone)]
pub struct AnthropicClient {
    http: reqwest::Client,
    api_key: String,
    auth_token: Option<String>,
    base_url: String,
    max_retries: u32,
    initial_backoff: Duration,
    max_backoff: Duration,
}

impl AnthropicClient {
    #[must_use]
    pub fn new(api_key: impl Into<String>) -> Self {
        Self {
            http: reqwest::Client::new(),
            api_key: api_key.into(),
            auth_token: None,
            base_url: DEFAULT_BASE_URL.to_string(),
            max_retries: DEFAULT_MAX_RETRIES,
            initial_backoff: DEFAULT_INITIAL_BACKOFF,
            max_backoff: DEFAULT_MAX_BACKOFF,
        }
    }

    pub fn from_env() -> Result<Self, ApiError> {
        Ok(Self::new(read_api_key()?).with_base_url(
            std::env::var("ANTHROPIC_BASE_URL")
                .ok()
                .or_else(|| std::env::var("CLAUDE_CODE_API_BASE_URL").ok())
                .unwrap_or_else(|| DEFAULT_BASE_URL.to_string()),
        ))
    }

    #[must_use]
    pub fn with_auth_token(mut self, auth_token: Option<String>) -> Self {
        self.auth_token = auth_token.filter(|token| !token.is_empty());
        self
    }

    #[must_use]
    pub fn with_base_url(mut self, base_url: impl Into<String>) -> Self {
        self.base_url = base_url.into();
        self
    }

    #[must_use]
    pub fn with_retry_policy(
        mut self,
        max_retries: u32,
        initial_backoff: Duration,
        max_backoff: Duration,
    ) -> Self {
        self.max_retries = max_retries;
        self.initial_backoff = initial_backoff;
        self.max_backoff = max_backoff;
        self
    }

    pub async fn send_message(
        &self,
        request: &MessageRequest,
    ) -> Result<MessageResponse, ApiError> {
        let request = MessageRequest {
            stream: false,
            ..request.clone()
        };
        let response = self.send_with_retry(&request).await?;
        let request_id = request_id_from_headers(response.headers());
        let mut response = response
            .json::<MessageResponse>()
            .await
            .map_err(ApiError::from)?;
        if response.request_id.is_none() {
            response.request_id = request_id;
        }
        Ok(response)
    }

    pub async fn stream_message(
        &self,
        request: &MessageRequest,
    ) -> Result<MessageStream, ApiError> {
        let response = self
            .send_with_retry(&request.clone().with_streaming())
            .await?;
        Ok(MessageStream {
            request_id: request_id_from_headers(response.headers()),
            response,
            parser: SseParser::new(),
            pending: VecDeque::new(),
            done: false,
        })
    }

    async fn send_with_retry(
        &self,
        request: &MessageRequest,
    ) -> Result<reqwest::Response, ApiError> {
        let mut attempts = 0;
        let mut last_error: Option<ApiError>;

        loop {
            attempts += 1;
            match self.send_raw_request(request).await {
                Ok(response) => match expect_success(response).await {
                    Ok(response) => return Ok(response),
                    Err(error) if error.is_retryable() && attempts <= self.max_retries + 1 => {
                        last_error = Some(error);
                    }
                    Err(error) => return Err(error),
                },
                Err(error) if error.is_retryable() && attempts <= self.max_retries + 1 => {
                    last_error = Some(error);
                }
                Err(error) => return Err(error),
            }

            if attempts > self.max_retries {
                break;
            }

            tokio::time::sleep(self.backoff_for_attempt(attempts)?).await;
        }

        Err(ApiError::RetriesExhausted {
            attempts,
            last_error: Box::new(last_error.expect("retry loop must capture an error")),
        })
    }

    async fn send_raw_request(
        &self,
        request: &MessageRequest,
    ) -> Result<reqwest::Response, ApiError> {
        let mut request_builder = self
            .http
            .post(format!(
                "{}/v1/messages",
                self.base_url.trim_end_matches('/')
            ))
            .header("x-api-key", &self.api_key)
            .header("anthropic-version", ANTHROPIC_VERSION)
            .header("content-type", "application/json");

        if let Some(auth_token) = &self.auth_token {
            request_builder = request_builder.bearer_auth(auth_token);
        }

        request_builder
            .json(request)
            .send()
            .await
            .map_err(ApiError::from)
    }

    fn backoff_for_attempt(&self, attempt: u32) -> Result<Duration, ApiError> {
        let Some(multiplier) = 1_u32.checked_shl(attempt.saturating_sub(1)) else {
            return Err(ApiError::BackoffOverflow {
                attempt,
                base_delay: self.initial_backoff,
            });
        };
        Ok(self
            .initial_backoff
            .checked_mul(multiplier)
            .map_or(self.max_backoff, |delay| delay.min(self.max_backoff)))
    }
}

fn read_api_key() -> Result<String, ApiError> {
    match std::env::var("ANTHROPIC_AUTH_TOKEN") {
        Ok(api_key) if !api_key.is_empty() => Ok(api_key),
        Ok(_) => Err(ApiError::MissingApiKey),
        Err(std::env::VarError::NotPresent) => match std::env::var("ANTHROPIC_API_KEY") {
            Ok(api_key) if !api_key.is_empty() => Ok(api_key),
            Ok(_) => Err(ApiError::MissingApiKey),
            Err(std::env::VarError::NotPresent) => Err(ApiError::MissingApiKey),
            Err(error) => Err(ApiError::from(error)),
        },
        Err(error) => Err(ApiError::from(error)),
    }
}

fn request_id_from_headers(headers: &reqwest::header::HeaderMap) -> Option<String> {
    headers
        .get(REQUEST_ID_HEADER)
        .or_else(|| headers.get(ALT_REQUEST_ID_HEADER))
        .and_then(|value| value.to_str().ok())
        .map(ToOwned::to_owned)
}

#[derive(Debug)]
pub struct MessageStream {
    request_id: Option<String>,
    response: reqwest::Response,
    parser: SseParser,
    pending: VecDeque<StreamEvent>,
    done: bool,
}

impl MessageStream {
    #[must_use]
    pub fn request_id(&self) -> Option<&str> {
        self.request_id.as_deref()
    }

    pub async fn next_event(&mut self) -> Result<Option<StreamEvent>, ApiError> {
        loop {
            if let Some(event) = self.pending.pop_front() {
                return Ok(Some(event));
            }

            if self.done {
                let remaining = self.parser.finish()?;
                self.pending.extend(remaining);
                if let Some(event) = self.pending.pop_front() {
                    return Ok(Some(event));
                }
                return Ok(None);
            }

            match self.response.chunk().await? {
                Some(chunk) => {
                    self.pending.extend(self.parser.push(&chunk)?);
                }
                None => {
                    self.done = true;
                }
            }
        }
    }
}

async fn expect_success(response: reqwest::Response) -> Result<reqwest::Response, ApiError> {
    let status = response.status();
    if status.is_success() {
        return Ok(response);
    }

    let body = response.text().await.unwrap_or_else(|_| String::new());
    let parsed_error = serde_json::from_str::<AnthropicErrorEnvelope>(&body).ok();
    let retryable = is_retryable_status(status);

    Err(ApiError::Api {
        status,
        error_type: parsed_error
            .as_ref()
            .map(|error| error.error.error_type.clone()),
        message: parsed_error
            .as_ref()
            .map(|error| error.error.message.clone()),
        body,
        retryable,
    })
}

const fn is_retryable_status(status: reqwest::StatusCode) -> bool {
    matches!(status.as_u16(), 408 | 409 | 429 | 500 | 502 | 503 | 504)
}

#[derive(Debug, Deserialize)]
struct AnthropicErrorEnvelope {
    error: AnthropicErrorBody,
}

#[derive(Debug, Deserialize)]
struct AnthropicErrorBody {
    #[serde(rename = "type")]
    error_type: String,
    message: String,
}

#[cfg(test)]
mod tests {
    use super::{ALT_REQUEST_ID_HEADER, REQUEST_ID_HEADER};
    use std::time::Duration;

    use crate::types::{ContentBlockDelta, MessageRequest};

    #[test]
    fn read_api_key_requires_presence() {
        std::env::remove_var("ANTHROPIC_AUTH_TOKEN");
        std::env::remove_var("ANTHROPIC_API_KEY");
        let error = super::read_api_key().expect_err("missing key should error");
        assert!(matches!(error, crate::error::ApiError::MissingApiKey));
    }

    #[test]
    fn read_api_key_requires_non_empty_value() {
        std::env::set_var("ANTHROPIC_AUTH_TOKEN", "");
        std::env::remove_var("ANTHROPIC_API_KEY");
        let error = super::read_api_key().expect_err("empty key should error");
        assert!(matches!(error, crate::error::ApiError::MissingApiKey));
    }

    #[test]
    fn read_api_key_prefers_auth_token() {
        std::env::set_var("ANTHROPIC_AUTH_TOKEN", "auth-token");
        std::env::set_var("ANTHROPIC_API_KEY", "legacy-key");
        assert_eq!(
            super::read_api_key().expect("token should load"),
            "auth-token"
        );
        std::env::remove_var("ANTHROPIC_AUTH_TOKEN");
        std::env::remove_var("ANTHROPIC_API_KEY");
    }

    #[test]
    fn message_request_stream_helper_sets_stream_true() {
        let request = MessageRequest {
            model: "claude-3-7-sonnet-latest".to_string(),
            max_tokens: 64,
            messages: vec![],
            system: None,
            tools: None,
            tool_choice: None,
            stream: false,
        };

        assert!(request.with_streaming().stream);
    }

    #[test]
    fn backoff_doubles_until_maximum() {
        let client = super::AnthropicClient::new("test-key").with_retry_policy(
            3,
            Duration::from_millis(10),
            Duration::from_millis(25),
        );
        assert_eq!(
            client.backoff_for_attempt(1).expect("attempt 1"),
            Duration::from_millis(10)
        );
        assert_eq!(
            client.backoff_for_attempt(2).expect("attempt 2"),
            Duration::from_millis(20)
        );
        assert_eq!(
            client.backoff_for_attempt(3).expect("attempt 3"),
            Duration::from_millis(25)
        );
    }

    #[test]
    fn retryable_statuses_are_detected() {
        assert!(super::is_retryable_status(
            reqwest::StatusCode::TOO_MANY_REQUESTS
        ));
        assert!(super::is_retryable_status(
            reqwest::StatusCode::INTERNAL_SERVER_ERROR
        ));
        assert!(!super::is_retryable_status(
            reqwest::StatusCode::UNAUTHORIZED
        ));
    }

    #[test]
    fn tool_delta_variant_round_trips() {
        let delta = ContentBlockDelta::InputJsonDelta {
            partial_json: "{\"city\":\"Paris\"}".to_string(),
        };
        let encoded = serde_json::to_string(&delta).expect("delta should serialize");
        let decoded: ContentBlockDelta =
            serde_json::from_str(&encoded).expect("delta should deserialize");
        assert_eq!(decoded, delta);
    }

    #[test]
    fn request_id_uses_primary_or_fallback_header() {
        let mut headers = reqwest::header::HeaderMap::new();
        headers.insert(REQUEST_ID_HEADER, "req_primary".parse().expect("header"));
        assert_eq!(
            super::request_id_from_headers(&headers).as_deref(),
            Some("req_primary")
        );

        headers.clear();
        headers.insert(
            ALT_REQUEST_ID_HEADER,
            "req_fallback".parse().expect("header"),
        );
        assert_eq!(
            super::request_id_from_headers(&headers).as_deref(),
            Some("req_fallback")
        );
    }
}